The invention relates to movable membrane devices for equalizing power levels in wavelength division multiplexed (WDM) systems.
Many fiber optic communications systems have been commercialized in which multiple wavelengths are used to carry the signals, thus increasing capacity. These are referred to as Wavelength Division Multiplexed (WDM) systems.
In a typical lightwave WDM system, especially those operating over long distances, the power level of the signal changes as a function of wavelength. This causes some channels to have higher power levels than others, and is known in the art as tilt. Excessive tilt is a serious disadvantage since the individual channels must be compensated for unequal power levels, which raises the cost of the overall system. Techniques for overcoming tilt in WDM systems have been devised. For example, Moving Anti-Reflection Switch (MARS) devices have been proposed for this function. According to this solution, two or more MARS devices, one having positive tilt and the other having negative tilt are used. By adjusting the air gaps of the two devices, power equalization can be obtained over the WDM band of the system.
The basic MARS device structure and operation are well known. The MARS device has a movable conductive membrane suspended over a conductive substrate. With an appropriate electrostatic field the membrane is controllably moved toward, or away from, the substrate thus producing a precisely controlled air gap between the membrane and the substrate. With proper positioning of the membrane with respect to the substrate the MARS device can be switched from a reflecting state to an anti-reflecting state. For more details of the MARS device see K. W. Goossen, J. A. Walker, and S. C. Arney, xe2x80x9cSilicon modulator based on Mechanically-Active Anti-Reflection layer with 1 Mbit/sec capability for fiber-in-the-loop applications, xe2x80x9d IEEE Phot. Tech. Lett., vol. 6, pp. 1119-1121, Sept. 1994.
While this approach to power equalization is effective, other solutions may provide more design options and possibly simper devices.
An alternative power equalizer has been developed which uses a single movable membrane device, similar to the MARS device, to reduce or eliminate tilt in an optical WDM system. In contrast with the conventional MARS device, the device of this invention produces no change of reflectivity at the center of the wavelength range of the device with changes in air gap. This is achieved by including only layers which are a half optical wave thick (at the center wavelength) in the membrane. Adding one or more half optical wave layers in a dielectric stack results in no change in the reflectivity of that stack at the center wavelength. Therefore at the center wavelength in the device of this invention, the membrane is practically invisible and therefore the reflectivity of the device is independent of air gap. However, at wavelengths less than or greater than the center wavelength, the membrane is visible, and therefore changes in air gap do result in changes in device reflectivity. Since the reflectivity changes are opposite in sign depending on whether the measuring wavelength is less than or greater than the center wavelength, the device produces tilt in its reflectivity spectrum that is dependent on air gap and thus on the electrical bias of the device. Either positive or negative tilt may be produced. Although a plurality of designs including only half optical wave thick layers in the membrane result in this behavior, a specific embodiment is shown that maximizes available tilt while minimizing loss (maximizing reflectivity) at the center wavelength. Dynamic equalization can be achieved using a power spectrum analyzer and a feedback loop to the power level equalizer.